In chemical pulping, wood chips are digested with the aid of a chemical solution to solubilize a portion of the lignin therein effecting the removal of the lignin. Typically, these digestive procedures comprise sulfite, sulfate (Kraft), soda and modified sulfite processes. Digestion takes place either in a continuous mode or in a batch digestion process.
Once the wood chips have been digested, the resulting pulp material is generally dark coloured cellulose fibres, namely brown stock. This material is called unbleached pulp. The dark colour means that not all of the lignin has been removed during digestion and that the remaining lignin has been chemically modified. The pulp suspension coming from the digestion process contains dissolved organic material and used inorganic cooking chemicals. The goal in brown stock washing after the digestion is to recover as much dissolved organic substance as possible for energy production and as much of the valuable inorganic chemicals as possible for regeneration with a minimum amount of dilution.
By removal of lignin the wood fibers are allowed to separate from each other in order to enable the formation of a fiber web. The bleaching is needed to further increase the brightness of the pulp. After washing the first bleaching stage is typically oxygen delignification, where oxidized white liquor or/and sodium hydroxide is used for delignification. In the modern fibre lines knotting, screening and washing are performed after the oxygen delignification. The filtrate from the washing after oxygen delignification is used as a washing liquid in brown stock washing i.e. counter-current washing.
Pulp from digestion process and oxygen delignification inevitably contains unwanted solid material. Some of the chips may not have been cooked properly, and some of the fibrous material may not be completely in the form of individual fibres. Contaminants other than wood may also enter cooking together with the chips. The purpose of knotting i.e. removal of knots and screening is to remove impurities and unfiberized particles from the main pulp stream to reduce the bleaching chemical consumption. In older fibre lines or if vacuum filters and wash presses are used in washing, the knotting is done after the digestion process.
Subsequently, the bleaching is often a multi-stage process employing chlorine, chlorine dioxide, oxygen, ozone, hydrogen peroxide, sodium hydroxide, peroxy acids, enzymes or a mixture of thereof. Still today, chlorine dioxide is a preferred choice for the bleaching chemical in many countries. Chlorine dioxide process has several advantages such as good bleaching capacity compared to e.g. hypochlorite, and it provides good strength and brightness values for the resulting pulp. Despite of its extensive use the chlorine dioxide process is not without drawbacks. Even though the price of chlorine dioxide is competitive compared to several other chemicals the equipment costs and loading on water systems cause pressure to decrease its consumption or even to find alternative bleaching solutions.
The reasons for the great consumption of the bleaching chemicals comprise side reactions wherein the chemical is consumed without any effect on the pulping product quality. For example, heavy metals bound to carboxylic acid groups tend to increase the bleaching chemical consumption by decomposing the chemicals, as well as chemical constituents formed or liberated during pulping, such as hexenuronic acid groups, which react further with the bleaching chemical.
Several known methods are discussing the removal of metals by e.g. the use of chelating agents or complexing agents of various types. Complexing agents of tertiary amine class such as DTPA (diethylene triamine pentaacetic acid), have been used in pulping processes for removal of metals from certain pulping stages, such as the oxidizing stage, in order to provide suitably long reaction times for the oxidizing agents such as hydrogen peroxide the decomposition of which the metals tend to catalyze.
In pulping most of the lignin is removed in digestion and only a small portion, a few percent, remains in the pulp slurry. In order to further reduce this remaining lignin content considerable amount of bleaching chemicals is required in proportion to the amount of the residual lignin. This suggests that the presently applied bleaching methods or chemicals are nonetheless inefficient.
Hexenuronic acid is formed during Kraft pulping through modification of the hemicellulose constituent in wood. During bleaching, the electrophilic intermediate of chlorine dioxide, such as hypochlorous acid, is readily reacting with hexenuronic acid resulting in increased consumption of the bleaching chemical. Typically, the removal of hexenuronic acid from pulp prior to bleaching is carried out using the process of selective acid hydrolysis which results in conversion of hexenuronic acid into 2-furoic acid, formic acid and 5-carboxy-2-furaldehyde. Unfortunately, the acid hydrolysis process is a time consuming step.
EP0786029B1 discloses a method for removal of hexenuronic acid in cellulose pulp by heat treatment. In this method cellulose pulp manufactured by sulfate or alkaline process having a kappa number less than 24 is treated at a temperature from 85 to 150° C., at a pH of 2-5 for a period of time from 5 min to 10 h. This treatment results in removal of at least 50% of the hexenuronic acid groups and decreases the kappa number by 2-9 units.
WO87/03313 discloses a process for bleaching pulp comprising the use of a premixture based on hypochlorous acid and further comprising a halogen salt, chlorine dioxide and/or a nitrogen compound, such as hexamethylenetetramine. This premixture is added to the pulp for minimizing fiber degradation and enhancing brightness. The bleaching time for this type of bleaching composition is 2-4 hours and the optimum pH range is from 2 to 6.
U.S. Pat. No. 4,298,428 discloses a method for improving chemical pulp strength and brightness by using hypochlorite bleaching wherein the pulp slurry further contains 0.01-0.5% by weight of an amine compound selected from the group consisting of ethylamine, methoxypropylamine, monoethanolamine, n-butylamine, 3-aminopropanol, isopropylamine, ethylenediamine, aniline, 2-amino-2-methylpropanol, 2-aminopropanediol, triethylenetetramine, 1,3-diaminopropane, diaminohydroxypropane, ethylenediamine, hexamethylenediamine and morpholine.
The efficiency of the presently used bleaching chemicals needs to be improved in order to avoid the typical delay times of several hours in a bleaching step. The consequence of the slow reaction rates and long delay times contributes to the huge size of the used reactors. Another drawback is the low selectivity of the reactions taking place. Valuable bleaching chemical is consumed in side reactions competing for the same chemical and not for the removal of hexenuronic acid. Moreover, more efficient recycling enabling a better closed circulation would be highly desired and beneficial for diminishing the environmental impact due to bleaching.